Pneumoconiosis ("Black Lung") Obligations (Details) - USD ($)		12 Months Ended
	Feb. 21, 2020	Dec. 31, 2021	Dec. 31, 2020	May 31, 2020	Apr. 01, 2016
Self Insurance Reserve Disclosure [Abstract]
Deductible amount per black lung claim		$ 1,000,000		$ 500,000
Collateral securities					$ 17,000,000
Increase in collateral securities	$ 39,800,000
Surety Bonds		17,000,000
Short-term investments		8,500,000
Black lung trust		2,600,000	$ 3,000,000
Estimated total black lung liabilities		37,100,000	36,900,000
Black lung obligations, current		2,600,000	2,400,000
Black lung obligations, noncurrent		$ 34,482,000	$ 34,567,000